The answer, it turned out, was Yen Ta Fo.
The unique color comes from the sauce that is stirred into the soup. It is slightly sweet and slightly tangy, and can vary in color from natural looking tomato-sauce red to almost flourescent hot pink.
The soup itself has rice noodles, kale, slices of fish cake, fish balls, fried garlic, and fried wontons. It's a nice, filling lunch but it isn't too heavy. I've been trying this whenever I see a cart that makes it, and while some versions can be a little bland, I really like all the different components that are mixed in.